Circuit breakers provide automatic power interruption to an electrical load when abnormal current conditions occur, such as an overload current or a short circuit. A circuit breaker is typically deployed on a line conductor between a load and a power source. The power source provides current over the line conductor through the circuit breaker and to the load, which is typically also connected to ground. A neutral conductor may provide a return path from the load through the circuit breaker and back to the power source.
A typical circuit breaker has a load terminal for connecting the circuit breaker to the load and a line terminal for connecting the circuit breaker to the line conductor. A braided wire conductor carries current from the line terminal to one side of a trip mechanism, the other side of the trip mechanism being connected to the load terminal by electrical contact to form a path for current to flow through the circuit breaker. When an abnormal current condition is detected, the trip mechanism automatically breaks the electrical contact with the load terminal to interrupt the flow of current.
In most circuit breakers, the trip mechanism uses a spring-biased trip lever to break the electrical contact with the load terminal. This trip lever is generally U-shaped and is retained in a latched position by an armature. The armature has a central opening within which the tip of the trip lever is seated. As long as the trip lever remains engaged to the armature, current is allowed to flow. When an abnormal current condition occurs, the armature is moved in a direction away from the trip lever such that the tip of the trip lever becomes disengaged from the armature. This releases the trip lever and breaks the electrical contact with the load terminal.
Moving the armature away from the trip lever is typically accomplished using a yoke to which the armature is connected. In electromechanical yokes, a bimetal strip bends in response to an overcurrent flowing through the bimetal strip. This bending of the bimetal strip distorts the yoke and forces the armature away from the trip lever. An electromagnetic yoke, on the other hand, has the braided wire conductor wrapped as a coil wound around the yoke. When a short-circuit current flows through the braided wire conductor, the yoke becomes highly magnetized and pulls the armature away from the trip lever. Many circuit breakers provide protection from both overcurrent and short-circuit current, which is typically much higher and can render the circuit breakers unfit for further use.
In most circuit breakers, the armature only needs to travel a small distance for the tip of the trip lever to come out of the central opening. Similarly, the trip lever only needs to travel only a small distance to break the electrical contact with the load terminal. Nevertheless, these circuit breakers allow the armature and the trip lever both to travel freely until they are otherwise stopped by other components in the circuit breakers. This requires extra space to be allocated within the circuit breaker to accommodate the excess movement. Such allocation of extra space is wasteful and inefficient, especially in miniature circuit breaker (“MCB”) where the goal is to reduce the size of the circuit breaker as much as possible.
In addition, due to their small size, many MCBs include a trip flag that helps visually to indicate when the device has been tripped. The trip flag is typically painted in an appropriate emergency color, such as orange or red, and is “pinned” or otherwise attached to the trip lever. When the circuit breaker is tripped, the movement of the trip lever also moves the trip flag into a viewing window in the outer casing of the circuit breaker to provide a visual indication the circuit breaker has been tripped. Existing miniature circuit breakers, however, use a trip flag that is a separate and discrete component. This increases the component count of the circuit breaker and complicates its assembly.
Accordingly, a need exists for an improved trip mechanism for a circuit breaker, and particularly a trip mechanism that allows the circuit breaker to be miniaturized as much as possible while also reducing the component count and assembly complexity of the circuit breaker.